Decoding Dusty Disks

The Spitzer and Hubble Space Telescopes each have found dusty disks around sun-like stars. Finding such rings of dusty debris around stars the size of the sun is difficult, because they are fainter and harder to detect than disks around more massive stars.

Young stars are born from clouds of gas and dust. Remnants of this cloud eventually produce gas giant planets like Jupiter and Saturn, while dust particles in the inner solar system collide, stick together, and form terrestrial planets like Earth. During this phase of planet formation, collisions between rocky bodies throw out dusty debris, creating a donut-shaped disk around the star. Over time, the planets orbiting the star sweep up most of this dust, so that only an outer ring remains.

Spitzer does not actually see distinct images of the disks. Instead, it detects the heat of material orbiting the stars.

"The amount of the heat they’re emitting is more than can be emitted by the surface of the stars themselves," says Beichman, who is the lead author of the Spitzer study. "The simplest explanation for this heat is a large reservoir of dust, orbiting tens of astronomical units (AUs) out, heated to about -350 degrees Fahrenheit. This material is analogous to the Kuiper Belt in our own system."

The Kuiper Belt is a region in the outermost portion of our solar system that contains comets and dust. Scientists believe it is a remnant of the dusty disk that once surrounded our sun. The presence of the Kuiper Belt is only inferred indirectly, and it wasn’t until Kuiper Belt Objects were discovered in the past decade that scientists were able to extrapolate how much dusty material might be out there.

Artist’s rendition of Kuiper belt.Credit: David Jewitt

Of the six stars that Spitzer found to have dusty disks, five are about the same age as our sun, ranging from 3 to 5 billion years old. One of the stars is only a few hundred million years in age.

All 26 of the stars analyzed by Spitzer were known to have planets. The gas giant planets orbiting these stars were discovered by radial velocity measurements, which determine how stars are affected by the gravity of an orbiting planet. Over the course of an orbit, a planet will pull at the star from different sides. As a star is pulled away from us, the starlight is Doppler-stretched to longer red wavelengths. When the star is pushed toward us, the starlight is scrunched toward shorter blue wavelengths. This light shift points to characteristics of the orbiting planet. So far, this technique has only been able to find massive gas planets.

Sequence of disk formation. Image Credit: NASA/Spitzer

Rocky planets could be present in at least one of the solar systems observed by Spitzer. The star HD33636 could have terrestrial planets in the inner solar system, because the gas giant detected in that system is 4 AU away from its star. The five other stars have gas giants much closer in, at 1 AU or less, making the presence of inner terrestrial planets less likely.

"I think the new disks discovered by Spitzer have the potential to be our Rosetta Stone," says Alycia Weinberger, staff research astronomer with the Carnegie Institution of Washington. "[They could] give us a translation between disks and whole planetary systems, to show us what the distribution of dust looks like in a system where we know the architecture of the planets."

Sequence of disk formation. Image Credit: NASA/Spitzer

Beichman says they are trying to figure out why only six of the stars had disks, but so far there is no obvious correlation between the disks and the planetary properties of each system. It could be that all 26 of the stars observed do have dusty disks similar to the Kuiper Belt, but they are too faint to be detected by Spitzer. So far, Spitzer has only been able to detect disks with 10 to 100 times more material than the Kuiper Belt contains.

The disks detected by Spitzer are 100 times fainter than dusty disks recently imaged by the Hubble Space Telescope. Hubble can only resolve disks with a lot of dust — approximately 1,000 to 10,000 more material than the Kuiper Belt has.

One disk seen by Hubble surrounds the young sun-like star HD 107146. This star is 50 to 250 million years old — old enough to have gas giant planets, but young enough to have rocky planets still being formed. Hubble also found a dusty disk around an even younger, smaller star: the 12-million-year-old red dwarf AU Microscopii.

The Hubble photos show a gap in the middle of both these disks. The disk around HD 107146 has an inner gap so big a solar system would fit inside it. This may indicate that any planets orbiting the star have swept up all the debris in the inner part of the dusty disk.